1. Field of the Invention
The present invention relates to a decompression device for an engine to reduce the compression pressure by opening at least one of an intake valve and an exhaust valve during the compression stroke at a low engine speed when the engine is started.
2. Description of the Related Art
A decompression device to reduce loads when starting the engine by opening at least one of the intake valve and the exhaust valve to maintain a combustion chamber in a half-compression state (a decompression state) to reduce the load when starting the engine has been known. A technology to release the decompression state by extending and opening a flyweight utilizing centrifugal force by rotation of a camshaft of the engine has been widely employed in decompression devices, and in these kinds of decompression devices, the flyweight must be urged to an angle-closing side by a predetermined urging force.
A return spring has been extensively employed to urge the flyweight to the angle-closing side, and in Japanese Unexamined Patent Application Publication No. 9-79017, a technology is disclosed in which the return spring constituted of a torsion spring is concentrically supported with a shaft part (a decompression cam) of the flyweight, thereby urging the flyweight to the angle-closing side by the urging force of the torsion spring. Furthermore, in Japanese Unexamined Utility Model Application Publication No. 6-58110, a technology is disclosed in which the return spring constituted of a coil spring is disposed in a gear part of the camshaft, and the flyweight is urged to the angle-closing side by the urging force of the coil spring.
In the decompression device, it is required that the flyweight is reliably turned to the extending and opening position at a predetermined engine speed forming the decompression release revolution speed, and the flyweight is reliably maintained at the angle-closing position when such as the engine is stopped. In this case, the moment provided to the flyweight by the urging force of the return spring is changed in a substantially linear manner as the engine speed is increased (in other words, the flyweight is extended and opened), and in order to simultaneously satisfy these requirements by the above technology, the spring constant of the return spring must be set to be small, and the tension provided to the return spring at the angle-closing position of the flyweight must be set high (refer to a two-dot-chain line in FIG. 3).
However, if the tension provided to the return spring at the angle-closing position of the flyweight is set to be high, the turning angle from the angle-closing position to the extending and opening position of the flyweight can be limited due to the characteristic of the return spring, and it is not preferable to continuously provide high tension to the return spring.